Using The Rat Grimace Scale to Detect Orofacial Pain in Mechanically-induced Temporomandibular Joint Pain in Rats

Introduction: Orofacial pain in animal models of TMJ disorders is typically evaluated by measuring evoked reflexive responses. Since the rat grimace scale (RGS) was adopted recently to assess spontaneous pain in other pathologies, this study evaluated its effectiveness for TMJ pain in the rat. RGS was evaluated using a well-defined pain model of TMJ loading. Material and Methods: Female Holtzman rats were assigned to separate groups: loading (n=10); sham (n=4); loading with naproxen (n=4) or vehicle (n=3) on days 4 and 5 after pain developed. Jaw loading was imposed for 7 consecutive days under anesthesia by repeated mouth-opening for 1hr. Sham had no mouth-opening. Naproxen or vehicle (1mg/kg) was given intravenously. Rats were videotaped for 30mins daily after loading, and for 7 days after loading was stopped. Images were randomized and quantitatively scored using 4 action units: orbital tightening, nose/cheek flattening, ear change, whisker change. The RGS score was compared between groups using a repeated-measures ANOVA and Tukey's post-hoc test. Results: Loading induced significantly higher (p<0.001) RGS scores than sham on days 1 and 5. After loading was stopped, RGS scores returned to sham levels for the remainder of test days. Naproxen injection significantly lowered (p<0.001) RGS scores from loading alone on day 7. Conclusion: Orofacial pain can be detected by the RGS, which may provide a useful new method to evaluate TMJ pain.